One of the advantages of MRI systems is a lack of moving parts. In contrast to CT imaging systems, there are no detectors or radiation sources which translate and/or rotate about the object or patient. Instead of the rotating parts, slices to be images are selected by manipulating field gradients and pulse sequences to provide images in the usual sagittal, coronal or transverse (axial) planes. Thus it is known to vary the static magnetic field with gradient pulses applied during the application of RF (Larmor) frequency pulses to select the imaging planes.
For example, consider an MRI system laid out according to the X, Y, and Z cartesian coordinates with the static field applied extending in the direction of the Z axis. The patient is oriented longtitudinally coaxially with the Z axis. Generally speaking, sagittal, coronal or transverse imaging planes are selected by applying the RF pulse simultaneously with the Y gradient pulse, an X gradient pulse or a Z gradient pulse, respectively.
In contrast to this, in other modalities using equipment such as CT scanners, the detector and/or the X-ray source are rotated. Usually rotation and thus data acquisition is accomplished for mechanical reasons in a transverse plane, about the patient's body. It is true that the source of X-ray energy and/or the detectors can be made to swivel so as to image planes at angles to the transverse planes. Nevertheless, the imaging capability is certainly mechanically limited. With MRI systems there is no such mechanical limitations; and therefore, theoretically, it is possible to acquire image data from any direction or in any plane. However, instead of mechanical limitations, there are practical, mathematical and processing limitations to obtaining images in non-orthoganal planes. Accordingly, such images in the non-orthoganal planes have not been used. Those skilled in the art know that when more than one gradient is simulateously applied during the excitation procedure, the imaging process will be unduly complicated. The actual gradients and the read or data collect gradient will also have to comprise multiple gradients. The selection of each of the gradients is further complicated by its relationship to the other gradients.
Certain set procedures are used in the prior art to obtain the exact type of image wanted in MRI systems. For example, the procedure followed in obtaining spin echo images in an orthogonal plane is to apply a plane selection gradient during the application of a shaped, selected RF saturation pulse signal. The position plane selection gradient is followed by a defocusing negative of the plane selecting gradient. After a time period "Ta" following the saturation RF pulse, an inversion RF pulse is applied. Again, a plane selecting gradient is applied during the echo inducing signal application period. Prior to the application of the read gradient, an encoding gradient signal is applied. The field generated by the encoding gradient signal is mutually orthogonal to each of the fields generated by the plane selecting gradient and the read gradient. After another time period Ta another inversion pulse is applied during the application of the plane selecting gradient pulse. A second echo pulse is generated at a time period Ta following the application of the second inversion pulse. The process is repeated as long as meaningful "echos" are obtained.
To select a non-orthogonal plane (herein a plane neither parallel nor normal to more than one of the orthogonal XY, XZ and YZ planes) in the object, at least two orthogonal gradient pulses have to be a simultaneously applied. Consequently, the encoding and reading gradients each also require at least two simultaneously applied gradient fields of proper amplitude and width to select the encoding and reading gradients. The complications involved in such a method have deterred imaging in any but non-orthogonal planes until now. Accordingly, there is a need for the efficient imaging of non-orthogonal planes with NMR equipment.
It is known in the X-ray medical imaging art to use means for visibly indicating the location on the patient of the imaging slices. Thus, for example, in CT imaging, it is known to apply external light lines to the patient for aiding in aligning the imaging equipment. See for example U.S. Pat. No. 4,385,397. Such indicating means have not been applied in the magnetic resonance imaging art.